Sources of n-alkanes in an urbanized estuary: Insights from molecular distributions and compound-specific stable and radiocarbon isotopes

Diagnostic molecular ratios and compound-specific C-13 and C-14 analyses were used to identify n-alkane sources in surface sediments collected along a transect from an urbanized estuary draining a peat-rich catchment (Tyne, UK). The most abundant homologues were generally C-29 or C-31, and the carbon preference index (CPI; 1.8 to 6.4) and average chain length (ACL; 28.5 to 29.5) of C-25-C-33 n-alkanes became progressively lower in samples closer to the mouth of estuary. delta C-13 signatures of C-19-C-31 n-alkanes ranged from -37.1 to -29.3 parts per thousand. and in general became more depleted with increasing carbon number. Delta C-14 values for C-21 (-945 to -738 parts per thousand) were significantly more depleted compared to C-29 (-591 to -65 parts per thousand) and C-31 (-382 to -96 parts per thousand), pointing to a much higher component of fossil (i.e., C-14-free) carbon in the shorter homologue. The radiocarbon contents for these three n-alkanes decreased toward the North Sea, which in conjunction with an up to 4 parts per thousand seaward C-13-enrichment in C-29 and C-31 and seaward decreases in CPI and ACL pointed to petrogenic hydrocarbon contamination in lower estuarine sediments. Independent C-13 and C-14 mass balances used to calculate the relative proportions of modern (i.e., plant wax) and fossil (i.e., petrogenic) n-alkanes yielded similar results and demonstrated that mixing with marine-derived organic matter (OM) or microbial degradation during estuarine transport led to a seaward decline in modern, longer-chain homologues, resulting in an increasingly larger fossil carbon contribution toward the mouth of estuary. The ability to clearly distinguish anthropogenic from natural inputs suggests that compound-specific radiocarbon analysis can successfully delineate the age of terrigenous OM delivered to the coastal zone even near historically polluted systems such as the Tyne. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.